1. Field of the Invention
The present invention relates to charge coupled devices (CCD) and in particular to the use of such devices for creating a charge packet which is equal to the difference of two input charge packets.
2. Background Art
In the technology of charge coupled devices various techniques have been used to do the arithmetic operation of subtraction. One such example of CCD subtraction is disclosed in U.S. Pat. No. 4,085,441 entitled "Monolithic Implementation of a Fast Fourier Transform" issued Apr. 18, 1978 to John L. Fagan on an application filed Nov. 24, 1976 and assigned to Westinghouse Electric Corporation. The patent issued to Fagan describes a CCD subtractor structure where two input signal voltages are subtracted one from the other using a charge-skimming technique. The resulting charge packet is representative of the difference between the two input voltages and is then converted into an output voltage. However, this structure utilizes input voltages and would not be compatible where discrete input charge packet subtraction is required.
In applications where discrete charge packets are to be subtracted, the most common technique requires the discrete charge packets to be converted into voltages so that the subtraction operation may be performed. An example of the technique of converting charge packets into voltages for subtracting is disclosed in U.S. Pat. No. 4,104,543 entitled "Multichannel CCD Signal Subtraction System" issued Aug. 1, 1978 to Dale G. Maeding on an application filed Feb. 22, 1977 and assigned to Hughes Aircraft Company. The patent issued to Maeding discloses a multichannel CCD structure which permits input signal charge packets to be subtracted one from another by alternately passing them under a common, periodically clamped floating electrode. The voltage assumed by the unclamped floating electrode is the difference between the charges. Hence, an output voltage represents the difference in the quantity of charge in two inputs signal charge packets. To obtain an output signal charge packet representative of the difference between the two input signal charge packets, the output voltage must be reconverted back into a charge packet.
For computationally intensive applications, such as that which might be employed in a focal plane imaging device for image preprocessing, a premium is placed on computing accuracy and real estate consumption. For such applications, the approaches described above for charge packet differencing are cumbersome and undesirable.
In many applications it is preferrable to directly subtract a first input charge packet from a second input charge packet with a resultant charge packet being the difference between the two input charge packets. An example of such a CCD subtractor is disclosed in U.S. Pat. No. 4,239,983 entitled "Non-Destructive Charge Transfer Device Differencing Circuit" issued Dec. 16, 1980 to Edwards et al on an application filed Mar. 9, 1979 and assigned to International Business Machines Corporation. The patent issued to Edwards et al discloses a CCD subtractor circuit wherein input charge packets are substracted using a floating gate electrode structure connected to a common node and diffusion. The input charge packets are represented as a separate pair of spatially separate charge packets which in turn represent positive and negative algebraic values using charge carriers of the same polarity. A disadvantage of such a subtractor circuit is that discrete charge packets must be broken into spatially separate charge packets. Directly inputting two charge packets would result in an output charge packet that would be the addition of the two input charge packets and not the difference.
It is therefore, an objective of the present invention to provide a CCD device for precise differencing of charge packets by using a gate charge subtraction technique which enhances the accuracy and linearity of the resultant charge packet.
It is another objective of the present invention to provide a CCD device which in addition to being a charge subtractor can be used as a charge packet replicator.
A further objective of the present invention is to provide a CCD device which can be used as a charge packet amplifier (or attenuator), individually, or in combination with the subtractor or replicator functions.